This invention relates to thin film solar cells. More specifically, this invention relates to thin film amorphous silicon solar cells.
Photovoltaic devices, such as solar cells, are capable of converting sunlight into usable electrical energy. The electrical energy conversion occurs as a result of what is well known in the solar cell field as the photovoltaic effect. Solar radiation impinging on a solar cell is absorbed by the semiconductor layer which generates electrons and holes. The electrons and holes are separated by a built-in electric field, for example, a rectifying junction, such as a P-I-N junction, in the solar cell. The electrons flow toward the N-type region and the holes flow toward the P-type region. The separation of the electrons and holes across the rectifying junction results in the generation of an electric current, known as the photocurrent, and an electric voltage, known as the photovoltage.
For maximum conversion efficiency of sunlight into electric energy, the semiconductor material should have a bandgap energy of about 1.5 electron volts. Photoactive intrinsic hydrogenated amorphous silicon based alloys, such as hydrogenated amorphous silicon, a-Si:H, or hydrogenated-fluorinated amorphous silicon, a-Si:F:H, has a bandgap energy, depending upon the method of fabrication, greater than about 1.7 eV. Doped hydrogenated amorphous silicon alloys incorporating P-type dopants have a bandgap energy which is closer to or below the ideal bandgap energy. However, the doping increases the defect state density of the material. U.S. Pat. No. 4,217,148, incorporated herein by reference, teaches adding a sufficient amount of a P-type conductivity modifier, such as boron, into the intrinsic amorphous silicon which is slightly N-type, to make the material less N-type and closer to a neutral state. However, incorporation of just enough P-type dopants to render the material intrinsic does not lower the bandgap of the material. Thus, it would be highly desirable to have a compensated cell wherein the intrinsic region had a lower bandgap which was more closely aligned with the optimum bandgap energy for absorption of solar radiation. It would also be desirable to have a cell wherein the greater absorption of the photoactive intrinsic region permits the fabrication of cells with a thinner active region and thus lowers the cost of the device and speeds the processing time. Furthermore, it would be desirable to have a compensated material fabricated by a method wherein the defects introduced during the doping process are offset by the increased absorption of the material due to the lower bandgap energy.